In the mechanical ventilation of a premises, the air is supplied to the premises by means of high impulse devices or by means of low impulse devices. The term high impulse device is taken to signify impulse devices which emit air to the ventilated area at high speed, while the term low impulse device is taken to signify devices which emit air at low speed. High impulse devices are employed in mixing ventilation, while low impulse devices are employed in displacing ventilation.
In mixing ventilation, a sufficient quantity of outdoor air is admixed so that the indoor air will be sufficiently pure. In this instance, a dilution of the indoor air will be obtained, and a uniform distribution of air pollutants present in the room.
In displacing ventilation, the air supplied by the low impulse device (the supply air) flows out over substantially all accessible parts of the floor of the room and forms an air stratum, as a rule at a height of the order of between 0.1 and 0.3 m. The height of the stratum is principally determined by the temperature of the supply air in relation to the air temperature in the room, and by the flow rate when the supply air leaves the impulse device. The supply air is at a lower temperature than the air which is present in the room. The flow rate of the supply air out of the low impulse device is kept relatively low and is, as a rule, of the order of between 0.3 and 0.8 m/s. From the region adjacent to the floor, the air rises towards the ceiling of the room and is evacuated from the room by suction extractors located in or adjacent to the ceiling. On movement towards the ceiling, the air entrains air pollutants which are present in the room and also absorbs thermal energy. Thereby, the air in the room is purified of pollutants at the same time as the air temperature is kept at the desired level.
Low impulse devices are unsuitable for use for heating purposes, since, in order to realize heating, the supply air must be at a higher temperature than the existing indoor air. The higher temperature and the consequential low density of the supply air, in combination with the low aspiration speed, entail that the air, once it has left the device, relatively rapidly assumes a substantially vertical direction of flow. As a result, substantially only air pollutants or impurities in a region adjacent to the device will be conveyed off, while, outside this region, the temperature and level of pollutants in the indoor air are affected to but an insignificant degree. The wish, in situations in which low impulse devices are employed and heating needs arise, to be able to switch to heating of the ventilated room by means of warm supply air while at the same time maintaining a low concentration of impurities at the occupied zone (the level most proximal the floor) has hitherto defied ready solution. This despite the fact that displacing ventilation has been employed for the last 15-20 years, and, with time, been put to greater and greater use, in particular in industrial premises.